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Thread: Actinic focus

  1. #1

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    Actinic focus

    Definition by Merriam-Webster -

    Actinic focus: the focus at which the chemically most effective rays as distinguished from the visually most effective are brought together (as by a lens).

    As I understand it meniscus lenses and semi-achromats (and possibly others) are affected by this lack of coincidence of the chemical and visual rays, and this can cause incorrect focus in the captured image if no correction is applied.

    Traditionally the amount of correction required was said to be 1/40th of the focal length at infinity, but more at closer distances.

    I just came across an ingenious device in the reprint of the 'Journal of the Photographic Society, Volume 1, 1854', described in a letter to the editor. The correspondent states that he finds 1/30 to be the most satisfactory adjustment and says that he 'constructed a very simple and inexpensive instrument to determine at once this difference, at whatever length the focal distance of the visual rays might be found'. The device is 'based on the principle of a proportional compass'.

    He goes on to describe it - he obtained a 30" flat rule about an inch wide, which he split into to two half-inch wide pieces. He then fastened them together with a thumbscrew at the 15" mark (creating an 'X' shape), and cut 1" off both pieces of one half. The longer side he marked V and the shorter C.

    He focuses, then sets the V side of the compass to be the distance between ground glass and lens. Reversing the compass to the 'C' side he then racks in the lens to the shorter distance.

    This got me wondering, apart from meniscus lenses, which other types (and models, if you can be bothered) of lens would require actinic focusing? Are any of the 'modern', soft focus lenses affected? (Is this the reason for the 'focus on the tip of the nose' advice given for the 12" Kodak Portrait?)

    And how does this apply to different media? Isochromatic films yes, what about ortho, x-ray, other modern films and paper negs nowadays?

    Lastly - the use of a yellow filter is said to be an alternative correction for the coincidence of the rays, does anyone use a yellow filter specifically for this purpose, and how effective is it? (Yes, the obvious answer is 'try it and see' but I'm interested to hear the experiences of others).

    (I'll have to stop reading those Journals and the like, I think I'm going all Victorian.. )

  2. #2

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    Re: Actinic focus

    Okay, let's go...

    First, you only have to worry about "actinic focus" being different from the visual focus if you are using blue and/or UV sensitive materials. Modern panchromatic films are sensitive to the visual spectrum and, therefore, visual focus = actinic focus with these materials. And this is regardless of the lens being used.

    With things like collodion and other blue-sensitive emulsions, the problem still only becomes significant if you have a lens that is not well-corrected for chromatic abberation, i.e., one where the blue end of the spectrum focuses at a significantly different place than red and green. With some vintage lenses and meniscus single-element lenses, this is an issue with these materials. In the past, when lenses were less-well corrected and photographic materials were only blue-sensitive, correcting focus to compensate for this was more important than it is today. If you use modern lenses and pan film, there's no issue at all.

    Even orthochromatic materials are sensitive to green, where the human eye is very sensitive. If you're using a meniscus lens with ortho material, you might see a bit of focus discrepancy, but this could easily be remedied by focusing through a cyan filter, so that your eye is only seeing the same wavelengths as the film sees.

    There is the problem of overall out-of-focus/softness in images made with lenses that haven't been well color-corrected. When used with panchromatic film, there ends up being a range of focus; the blue end focusing in one plane and the red end in another, with everything else in between. With no filter and a multicolored subject, you'll have a range of focuses, even with one subject. Imagine something magenta, with a mix of red and blue, and it's edges focusing at two different planes and being rendered at two different sizes. This produces a halo typical of chromatic abberation. In this case, a yellow filter, which filters out the blue end, would improve overall sharpness, simply by eliminating one extreme and leaving only red and green, which focus more closely to one another.

    With older (or poor-quality) enlarging lenses and graded papers, which are only sensitive to blue light, there could be a discrepancy between actinic and visual focus. The usual remedy for this was to focus through a blue filter. A blue focusing filter was supplied with some grain magnifiers for just this purpose. Nowadays, enlarging lenses are generally well color-corrected, this is no longer a real issue, especially since most papers these days are VC and sensitive to green as well as blue.

    Keep in mind as well that the vast majority of photographic lenses pass very little UV. Glass is a poor transmitter of UV; lens cements as well. Plus, modern coatings do even more to limit the UV portion of a lens' transmission spectrum. Lenses made of special materials (like silica and calcium fluoride) instead of glass, and even plastic, are needed for UV photography. So, even if you have materials that are strongly UV sensitive along with being blue sensitive, it's only the visible blue (and maybe the near UV) that needs to be compensated for. If you, indeed, have a set-up with blue sensitive materials and a non-color-corrected lens, then applying a focus adjustment would be one way to compensate for the discrepancy. Using a blue filter would be another.

    Best,

    Doremus

  3. #3

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    Re: Actinic focus

    Thank you Doremus for that very comprehensive answer.

    I guess my interest here is in older lenses and trying to understand how better to use those that might be not be (intentionally or otherwise) very well corrected, particularly meniscus and soft focus (or 'pictorial') lenses. What you see is not always (if ever) what you get!

    Your comment about a multicoloured subject that has a range of focuses under these circumstances is interesting. I think I'll try a yellow filter, with the intention of taming the effect a little rather than killing it.

    Cheers,
    Peter

  4. #4

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    Re: Actinic focus

    Quote Originally Posted by peter brooks View Post
    ... Your comment about a multicoloured subject that has a range of focuses under these circumstances is interesting. I think I'll try a yellow filter, with the intention of taming the effect a little rather than killing it.

    Cheers,
    Peter
    Just don't use the yellow filter with blue-sensitive film/plates

    With pan film, you could probably use any of a number of filters to help eliminating the chromatic abberation of the lens and more closely match where "visual focus" is. A #15 yellow orange would cut more sharply than a #8 yellow and is also in the sweet spot of visual sensitivity. A #58 green would work too, but would likely be overkill (but would eliminate both blue and red, i.e., the extremes of focus discrepancy).

    In any case, you'll have to take the effect of the filter into account; if you like what it does for the image then great. Yellow has the least effect, darkening just blues, but stronger filters will make a larger change in the image.

    Best,

    Doremus

  5. #5
    Joe O'Hara's Avatar
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    Re: Actinic focus

    I was going to tackle that question but I have nothing to add to Doremus' excellent explanation.
    Where are we going?
    And why are we in this handbasket?


    www.josephoharaphotography.com

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